Safe and economic railway systems require the performance of periodic rail maintenance. As part of the maintenance, it is prudent that the rails be inspected for faults, that grinding machines maintain the rails in smooth, properly shaped condition to allow safe movement of rail traffic thereon, and that the rail geometry be ascertained to determine whether various rail parameters are within specified acceptable tolerances, or whether the rails should be condemned. To perform certain of these maintenance procedures, it is helpful to determine where the centerline of the web of the rail is located. In this regard, the web centerline is utilized as a reference point to properly grind the rail, to determine if flaws are at unsafe locations within the rail, etc.
Numerous approaches to detect rail web centerline have been proposed. For example, laser triangulation measurement devices aimed at both sides of the rail web have been used. Strobe lights in combination with image sensing cameras having mirrors either on the rail web, the foot area of the rail, or even under the head area of the rail have been utilized. The rail centerline has been detected by the use of magnetic proximity sensors positioned near the web or head of the rail to sense flux density. Web centerline has been located mechanically by hand gauging with the use of mechanical feelers on the web of the rail.
U.S. Pat. No. 4,235,112 to Kaiser is also known and discloses laterally movable ultrasound transducers, one transmit-receive transducer and two receive transducers, which are automatically centerable on the rail while a rail fault detection device is moved along the rail. In this regard, a sensor head is movable laterally with respect to a carriage. The carriage moves along the rail longitudinally. The transducers in the sensor head are acoustically coupled to the rail and electrical signals, generated by the transducers based on reflected ultrasound energy, are used to generate a position error signal. A hydraulic position control mechanism, in response to the position error signal, controls the lateral movement of the transducers. The transducers are positioned laterally to null the error signal.
Several problems exist with prior approaches to detecting web centerline of a rail. Under many circumstances, the rail web, feet, or underside of the head, are not available to be probed by strobe lights; nor are they available for use with magnetic sensors, laser triangulation devices, nor mechanical feelers. For example, in many circumstances the rail web, feet, and in many cases, portions of the head are covered with high ballast or snow. In addition, road surfaces at road crossings are substantially level with the rail head and often cover the foot and web of the rail. As indicated, such conditions render many of the previously mentioned approaches ineffective. However, devices such as disclosed in U.S. Pat. No. 4,235,112 to Kaiser which utilize ultrasound through the head of the rail, are still capable of functioning to locate web centerline.
The Kaiser device as best understood, locates web centerline of a rail only when it is positioned as in FIG. 4, wherein the rail's web centerline is substantially perpendicular to the ground surface. The problem is that rail is normally canted as shown in FIG. 5, that is, the rails are canted inward at a cant angle .theta. with regard to a perpendicular to the ground surface. This cant angle .theta. is usually a predetermined angle created by the mounting of the rail upon the tie plate and can be intentionally varied in any stretch of rail, such as for curves.
Because of the cant angle, movement of a single ultrasound transmit-receive transducers and two receive transducers coupled to the head of a rail in a lateral direction, as described in U.S. Pat. No. 4,235,112, is not effective to detect the true web centerline of the rail. That is, web centerline with the Kaiser device can only be located if the web centerline is perpendicular to the ground surface, as shown in FIG. 4.
In addition to not being able to locate web centerline of a canted rail, none of these devices appear usable under many circumstances, as mentioned above, to determine rail cant, rail height, rail height loss, gauge face loss, and/or track gauge. These measurements can be very important for various aspects of maintenance of railway systems. Because of these many shortcomings of the prior art, the need for the present detection apparatus has arisen.